1. Field of the Invention
This invention relates to methods and apparatus for image data compression.
2. Description of the Prior Art
In an image data compression system, often the output bit rate of a compressed data stream is constrained by the capacity of a transmission or storage channel. A particular example of this is when compressed video data has to be recorded on a tape channel; generally, to maintain real-time operation, a whole field or frame's worth of compressed data has to be recorded onto a predetermined number of record tracks.
In order to comply with this constraint, the bit rate of the output compressed data is controlled by controlling the degree of compression applied. In many cases, this is achieved through controlling a quantisation factor applied to the video data (e.g. after transformation to the frequency domain).
The effect of varying the quantisation factor on the quantity of compressed data is highly non-linear and not easily predictable. This means that a suitable quantisation factor can be chosen accurately only by performing trial compressions, and this in turn limits the resolution to which the quantisation factor can be controlled.
So, the quantisation value is not continuously variable; the way in which it has to be selected, by the trial-and-error method, means that it varies between discrete values, usually with a significant step between adjacent values.
Previously, the "safe" solution to selecting a suitable quantisation value has been to select the quantisation factor which gives a bit rate nearest to, but less than the required bit rate.
However, because of the step between adjacent available quantisation factors, this means that the actual bit rate is always less than the available bit rate. In one proposed system, on average only about 97% of the available bit rate is used; the remaining bit rate is lost as part of the process of making sure that the required bit rate was not exceeded.
It is an object of the invention to provide a data compression system making more efficient use of an available bit rate.